The long-range goal of this continuing project is to develop and apply the technology of functional magnetic resonance imaging (FMRI) to the study and characterization of neural mechanisms responsible for normal visual perception and attention. The project will concentrate on developing a set of neurophysiological principles that characterize the attention-related control of cortical visual processing in humans. (1,2) In a pair of initial experiments, two fMRI-based measures of cortical activity will be critically evaluated for their ability to accurately reflect the behavioral effects of attention. We will examine the ability of these measures to accurately depict the intensive and topographic properties of attention under three different types of attention-intensive tasks: absolute detection, increment detection and feature conjunction detection. We hypothesize that these measures will reflect the specific attentional demands of the tasks. (3) Next, these measures will be used to observe the "top-down" effects of volitional control of attention, first in isolation, then in conjunction with the bottom-up influences of single and multiple objects in the field of view. A novel method for visualizing the "attentional field" will be introduced. (4) In a fourth set of experiments, this approach will be used to test the hypothesis that attention topography can reflect both spatial and object-related characteristics of the visual display. The latter will be tested with occlusion, color, and stereoscopic cues for object segmentation to determine if the induced attentional topography is cue-invariant. (5) An experiment based on "attentional crowding" will be used to identify the cortical site (or sites) of attentional selection. (6) In a final study, we examine the effects of attending to different spatial reference frames (retinal vs object-oriented) on cortical activity. We hypothesize that occipital visual areas will be unaffected by the subject's reference frame but that portions of parietal and/or frontal cortex will be affected. Together, the results of these studies will significantly advance our understanding of the neurophysiological basis of visuospatial attention and the mechanisms controlling the access of visual information to conscious awareness.